SUMMARY The grass powdery mildew fungus, Blumeria graminis is classified into eight formae speciales (ff.spp.) based on strict host specialization. However, evidence suggests that host ranges extend to more than one genus and are particularly diverse among samples from the Middle East, the proposed centre of origin and diversification of crop plants. This study investigated whether geographical origin, host species or both determine the genetic variation in B. graminis that is found in cereals, sampled from Europe, Asia and North America, and whether there is any evidence for co-evolution between pathogen and host. Phylogenetic analysis of nucleotide sequence variation within the ribosomal DNA Internal Transcribed Spacer (ITS) regions and the beta-tubulin (tub2) gene gives rise to two dendrograms with different topologies. In both trees, isolates of B. graminis from cultivated cereals are grouped according to their principal host genus. This grouping was supported by amplified fragment length polymorphism (AFLP) analysis and cross-infectivity tests. However, there was no evidence of co-evolution. There was far greater divergence between ff.spp. in tub2 sequences than ITS regions and a faster rate of mutation of tub2, especially in the third base position of exons. It is proposed that variation in the rDNA-ITS regions is constrained either by their functional role in the processing of rDNA precursor molecules or by concerted evolution, hence limiting their use in phylogenetic studies. AFLP data suggests an overall lack of correlation between geographical and genetic distances. This may be related to the long distance dispersal exhibited by B. graminis.